Because you have both flaps working off of one channel, you don't have to use any mixers to use a 3-position switch. Just go into the AUX-CH section of the Basic Menu, and set channel 6 (Flaps) to a 3-position switch. Now your flaps will have 3 positions. You can go into the ATV section to set the endpoints.

Now move the switch to the 1/2-flaps position (the middle position). Set your servo linkage such that you have the amount of deflection you want with 1/2-flaps. Now move the switch to flaps 'up', and adjust the ATV of channel 6 so that the flaps are in the proper 'up' position. Toggle to flaps down and set the other endpoint.

Now just use a pmix with flaps as master and elevator as slave to get the elevator mixing. If you use pmix 6 or 7, you can fine-tune the exact points.